1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to a method of reading color image, color image reading apparatus and system for reading color image for obtaining image data by reading a color film.
2. Description of Related Art
An apparatus has been developed which optically reads a color film by an image sensor composed of a storage-type charge transfer device, such as a CCD, thereof and then separates the color of the image so as to obtain image data.
However, in a case where image data is obtained from a color film, the transmittance of the film varies depending upon the type of the film or exposing conditions employed in the photographing operation. Therefore, there arises a problem in that desirable color signals (three primary colors consisting of red, green and blue) cannot be obtained only by, as it is, A/D-converting an output from the CCD to determine the quantity of transmitted light as has been performed when a monochrome film is read.
As a technique attempted to solve the above-mentioned problem, a method is available which comprises the steps of performing scanning (prescanning) for analyzing exposing conditions employed in the photographing operation before scanning (main scanning) for obtaining image data from the color film is performed; and performing main scanning by using a result of the analysis so as to reproduce the gradation and/or color.
As an example of the above-mentioned method, a technique disclosed in Japanese Patent Publication No. 7-22311 is structured as follows:
(1) Initially, shading data is obtained such that an illuminating means illuminates an original color image document with a first quantity of light, and then a color image signal obtained by color-separating the image by a reading means is stored in a storage means.
(2) Then, exposure conditions and so forth employed in the photographing operation are detected to determine the quantity of light of the lamp by performing prescanning such that the illuminating means illuminates the original color image document with the first quantity of light, and then a color-separated color image signal is obtained by the reading means. Then, sampling of the density of the image of the color original document is performed so that each of R, G and B pixels in the color image signals for each sample line is subjected to a correction calculation for a standardizing process with shading data obtained in step (1). Thus, the white level of the color image signal is identified.
After reading and the correction calculation of all of the sample lines have been completed, the quantity of light (a second quantity of light) for performing main scanning is obtained such that a determining means determines the second quantity of light in accordance with the ratio of a predetermined value corresponding to an output from the reading means with respect to the white image and the above-mentioned white level in such a manner that the white level of the color image signal realized by irradiating and reading (the color image document) with the second quantity of light is made to be the above-mentioned predetermined value. Then,
(3) Main scanning is performed.
As another example of the above-mentioned method, a technique has been disclosed in Japanese Patent Publication No. 7-86652 in which data of a light transmittance characteristic for each color with respect to quantities of exposure of various color films previously obtained by measurement is previously stored in a memory means. Then,
(1) initially, prescanning is performed, and then the distribution of levels of image signals obtained by prescanning and then data of the above-mentioned light transmittance characteristic are used to determine an exposure range employed in the photographing operation. Then, a result of the determination is made to correspond to the gradient so as to obtain gradient characteristic conversion data for converting the gradient characteristic of each image signal obtained by main scanning.
(2) Then, main scanning is performed, and then the gradient characteristic of an image signal obtained by main scanning is converted in accordance with the foregoing gradient characteristic conversion data.
In a word, in a case where image data is obtained from a color image document, such as a negative film, the conventional technique has been structured such that the color image document is prescanned to obtain exposure conditions and so forth employed in the photographing operation. Then, the exposure conditions and so forth are used to perform main scanning so as to reproduce the gradient and the color. Among the foregoing techniques, the technique disclosed in Japanese Patent Publication No. 7-22311 is structured such that a portion (a base film) of the color image document is read with the first quantity of light before the exposure conditions and so forth employed in the photographing operation are detected so as to obtain shading data. Then, R, G, B for each pixel of the color image signal obtained by prescanning are corrected, and then prescanning is performed with the first quantity of light. The technique disclosed in Japanese Patent Publication No. 7-86652 is structured such that gradient characteristic conversion data for converting the gradient characteristic of each image signal obtainable by main scanning is obtained when prescanning is performed by storing, in the memory means, light transmittance characteristic data for each color with respect to the quantities of exposure of various color films obtained by previous measurement.
As described above, one of important factors to obtain image data from a color image document is to obtain the exposure conditions and so forth employed in the photographing operation. To obtain preferred exposure conditions and so forth, an appropriate quantity of the lamp must be obtained. In this case, influences of individual differences of the components of the scanner, such as the deterioration of the lamp and the difference in the sensitivity among the image sensors, such as the CCD, on the quality of the image cannot be ignored.
For example, the above-mentioned conventional technique has been arranged such that the quantity of light of the lamp preferable for the main scanning operation is obtained by determining the voltage with which the illuminating means (a projecting lamp) is turned on and the diaphragm ratio to perform the shading correction so as to absorb dispersion in the line direction. Thus, dispersion among the films can somewhat be absorbed. However, no contrivance for absorbing dispersion occurring attributable to individual differences of the components of the scanner has been disclosed.
Hitherto, dispersion occurring due to the individual differences have been overcome by providing a safety allowance for an allowable error when the product of the color image reading apparatus is manufactured. That is, in a case where the quantity of light stored in the sensor exceeds a saturation point or has a value near zero, the former case results in that the output from the sensor cannot be identified as an image signal. In the latter case, noise is made to be excessive, causing the quality of the image to deteriorate considerably. To solve the foregoing problems, design of the product has been performed such that the reference value for the stored quantity of light is set to a safety value near the central value to prevent the fact that the quantity of light stored in the sensor does not exceed the saturation point or have a value near zero.
However, it is ideal for realizing a high image quality to make the reference value for the stored quantity of light to approach the saturation point. Since many noises are generated in a direction toward zero, fixation of the reference value for the stored quantity of light to a value near the central value results in many noises being generated as compared with a case where the reference value is made to be a value near the saturation point. Therefore, the quality of the image in the data state has a limitation and thus an image having a high quality cannot easily be obtained.